Protective relays are devices designed to protect an electrical power system during disturbances. To accomplish their functions, the relays receive signals from the power system mainly by way of two types of devices, the potential transformer and the current transformer.
Potential transformers convert high voltage levels down to low voltage levels. Current transformers convert high current levels to low current levels. The lower level currents are then received by devices like protective relays and demand meters.
It is well known that current transformers have a unique operating characteristic. If the current transformer is operating under load—with current flowing in the primary side—and the low level's secondary side circuit is open, then high voltages will develop at the current transformer secondary terminals which can be dangerous or fatal to operators and may damage devices connected to the current transformer's circuits. Additionally, an open transformer circuit results in a disconnuity of signal provided to the protection and control equipment, such as the protective relays, causing likely misoperations or failure to operate. Moreover, an open circuit associated with a current transformer can result in magnetization of the current transformer that causes incorrect output even after the open circuit condition is corrected. In robust power systems, the current transformer is considered to be a weak link since there are not many ways to understand the overall health of the device other than off-line testing.
The prior art provides over voltage protection against the unplanned opening of current transformers by utilizing a test plug and meter. This is accomplished by continuous monitoring of the voltage levels on the secondary side of the transformer. If an over voltage condition develops, an electronic component that is placed in parallel with the main current path detects the condition. In response to the over voltage condition, the electronic component switches on, bypassing the open circuit and returning the current flow and eliminating the over voltage condition. Accordingly, the prior art device has the ability to limit the voltage to safer levels of under 100 volts.
However, this approach is still not considered to provide adequate safety protection. Accordingly, other devices have been developed that analyze the current flow. For example, when the alternating current flow crosses a zero value, the prior art electronic switching device turns off, eliminating the bypass of the open circuit and the current continues to flow in the primary (high side) of the current transformer. With a secondary side of the transformer open circuited, an over voltage condition again develops across the secondary and the protection on the test plug once again limits the voltage to safer levels and the cycle continues to repeat until the test plug is removed from the circuit. Thus, until the test plug is removed from the circuit, the user of the test plug is continuously exposed to voltages that may exceed desired safety levels. Unfortunately, the monitoring of the on and off cycles causes a lack of continuity of current to the protective relays and also negatively impacts the integrity of the current waveform as that waveform is distorted in the secondary side of the current transformer. This lack of either continuity or integrity in the secondary circuit could be seen by some protective relays as a disturbance in the electrical power system, thereby causing protective relays to take the power system out of service unnecessarily.
Another prior art solution provides for a test switch connected between the transformer's secondary side and a protective device. A plug for testing the circuit is inserted into the test switch so that the secondary side current flows through the test plug wherein the test plug comprises a circuit for closing the current transformer secondary side when the secondary side is open circuited and closing the circuit when closed, thereby maintaining a continuous flow of current through the protective device. Although this test plug configuration is an improvement, it is believed that a further improvement can be obtained by directly monitoring the voltage and/or current conditions of the secondary side of the current transformer thus allowing for the ability to instantaneously correct or generate an alarm when an open circuit condition is detected.